The Australian Government's decision to detain 157 Tamil asylum seekers at sea for nearly a month was illegal, lawyers say.

The validity of the law to detain the group in June is under the spotlight at the High Court in Canberra.

The court was today told by lawyers representing the asylum seekers that the group was illegally detained and denied procedural fairness when they were not asked about their refugee status.

The group, which includes 50 children, were intercepted at sea and taken aboard the Australian Customs ship the Oceanic Protector.

The group had set out from Pondicherry in India after fleeing Sri Lanka.

A failed attempt to return the group to India saw them transferred to Nauru via the Cocos Islands and the Curtin Immigration Detention Centre in Western Australia.

Government lawyers told the court the authorities that detained the group were upholding migration laws.

The Government also defended the Maritime Powers Act used to detain the group.

Commonwealth Solicitor-General Justin Gleeson told the court the law which allowed the group's detention at sea was deliberately coercive because it needed to apply to law enforcement in many different circumstances beyond the Migration Act.

In one of the earlier directions hearings, High Court Justice Kenneth Hayne noted the Act seeks to apply the law beyond Australian territorial waters and raised issues not considered before.

The Human Rights Law Centre's Daniel Webb said the outcome could have serious implications for the Government's asylum seeker policies.

"In particular, the court will look at the Government's power to detain people at sea and then take them elsewhere and consider whether that's a power that needs to be exercised fairly; whether that's a power that needs to be exercised with due consideration for individual circumstances," he said.

Greens senator Sarah Hanson-Young said the decision to detain the asylum seekers on the high seas was cruel and absurd.

"What the Government is trying to do is to give themselves a licence to breach international law and to throw away all decency and basic common sense," she said.

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